Systems applying a phase shift to a periodic signal are well known from the state of the art. Such systems may comprise in particular a phase shifter to which the periodic signal and a control signal are supplied. The phase shifter then shifts the phase of the received periodic signal by a value which is determined by the received control signal. The control signal can be a control voltage or a control current, and the applied phase shift is usually linearly dependent on the provided control signal.
For some applications, it is desirable to be able to tune the phase shift applied by a phase shifter to a periodic signal continuously over a very large range. The operating range of a phase shifter, however, is limited by a certain minimum shift and a certain maximum shift. In some cases, also the range of the control signal may be restricted.
The limits of the operating range of a phase shifter and of control signals can be circumvented by making use of the periodicity of a periodic signal. A periodic signal has a periodicity of 360° or 2π. Therefore, a periodic signal with the same phase angle is achieved by applying a specific desired phase shift to the signal or by applying this desired phase shift after a phase angle of an integer multiple of 2π is subtracted from or added to the desired phase shift. As long as the phase shifter covers at least a range of 2π, any desired phase shift can be achieved by a corresponding subtraction or addition. This corresponds basically to a modulo 2π operation. The control signal simply has to be changed accordingly, in order to be able to tune the phase shift applied by a phase shifter continuously over a large range.
It is a disadvantage of this approach, though, that the required control signal exhibits abrupt changes which are smoothened due to the inevitable finite bandwidth of any physical system. This problem will be illustrated in the following in more detail with reference to FIGS. 1 and 2.
FIG. 1 is a simplified block diagram of an exemplary system enabling the application of a phase shift to a periodic signal which is continuously tuned over a large range.
The system comprises a local oscillator 10 generating the periodic signal “LO” to which phase shifts are to be applied. The output of the oscillator 10 is connected to a phase shifter 11. The phase shifter 11 is able to apply phase shifts between −π and +π to an input periodic signal. The phase shifter 11 applies a respective phase shift as a linear function of a provided control voltage “PCon”, a control voltage of 3V corresponding to a phase shift of +π and a control voltage of −3V corresponding to a phase shift of −π. The output of the phase shifter 11 constitutes the output of the system.
Further, the system comprises a control voltage generator 14 which outputs a control voltage “In”. The control voltage generator 14 is connected via a processing unit 15 to a control input of the phase shifter 11. Between the processing unit 15 and the control input of the phase shifter 11, a lowpass filter 16 is indicated in addition. This lowpass filter 16 does not constitute a distinct component, but models the finite bandwidth of the system.
The operation of the system will now be described with reference to FIG. 2. FIG. 2 is a diagram depicting the waveform of three signals occurring in the system of FIG. 1.
The local oscillator 10 generates a periodic signal “LO” and provides it to the phase shifter 11.
At the same time, the control voltage generator 14 generates and provides a control voltage “In” representing the desired phase shift. In the current example, the control voltage “In” has the form of a decreasing ramp, indicating that the phase angle of the periodic signal is to be decreased continuously with a certain speed. The amplitude in Volt V of the decreasing ramp is depicted in FIG. 2 over a time span of 100 μs, the amplitude decreasing within this time span from 0V to −20V.
The control voltage “In” is converted by the processing unit 15 to an equivalent saw wave “OCon” by adding a voltage of 3V to the control voltage “In” and by further using a modulo 2π operation on the resulting increased voltage. That is, whenever the resulting increased voltage falls below −3V, additional 6V are added to it. The amplitude in Volt V of the saw wave “OCon”, which is equally depicted in FIG. 2 over a time span 100 μs, thus decreases from 3V to −3V and then jumps back to 3V before decreasing again.
The generated saw wave “OCon” is provided by the processing unit 15 to the control input of the phase shifter 11. Due to the finite bandwidth of the system, however, the real control voltage “PCon” fed to the phase shifter 11 corresponds to this saw wave “OCon” after subjection to a lowpass filtering represented by lowpass filter 16. The amplitude in Volt V of the real control voltage “PCon” is equally depicted in FIG. 2 over a time span of 100 μs. The difference between the generated saw wave “OCon” and the real control voltage “PCon” represents the distortion due to the finite bandwidth of the system.
The phase shifter 11 applies to the received periodic signal “LO” a phase shift which corresponds to the received real control voltage “PCon”. The distortion of the saw wave “OCon” thus appears also in the phase of the signal “Out” output by the phase shifter 11.